From the sounds of it you have one application set up that takes the hostname of domain entered and determines what site is being viewed based on that.
If that is the case there could be a lot of overlap with the content of each site, I would imagine. If so, it could be considered "duplicate content" to some extent, which could hurt your results in the search engines.
A lot of the written content would be the same, as would images. I would also assume that the URLs to most of the pages on the site would be the same as well regardless of which domain name was visited.
Here's what Google has to say (emphasis mine):
Duplicate content generally refers to substantive blocks of content
within or across domains that either completely match other content or
are appreciably similar. Mostly, this is not deceptive in origin.
Examples of non-malicious duplicate content could include:
- Discussion forums that can generate both regular and stripped-down pages targeted at mobile devices
- Store items shown or linked via multiple distinct URLs
- Printer-only versions of web pages